1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus, such as a copying machine, a facsimile machine, a printer and the like. The present invention particularly relates to an image forming apparatus provided with a transfer-convey device that transfers a toner image on an image bearing member to a transfer member and that holds and conveys the transfer member with a belt-shaped moving member (hereinafter referred to as a transfer belt).
2. Discussion of the Background
Typical steps of an electrophotographic image forming process used in many image forming apparatuses, such as copying machines, facsimile machines, and printers and the like are as follows:
charging a photoconductive element that serves as an image bearing member with a charging member; PA1 causing the photoconductive element to be directly exposed to light reflected from an original document image using an exposing device, or optically writing an image on the photoconductive element according to an image signal by the use of an optical writing system that adopts a laser scanning system, an LED (Light Emitting Diode) writing system, or the like, so as to form an electrostatic latent image on the photoconductive element; PA1 developing the latent image borne on the photoconductive element with toner particles contained in a developing device; and PA1 transferring the toner image onto a transfer member, such as a transfer sheet, a film such as a polyester film, or the like.
Thereafter, the transfer member holding the toner image thereon is conveyed to a fixing device. An image is obtained by means of the process as mentioned above.
In such an image forming apparatus, a transfer-convey device that includes a transfer belt has been widely used. The transfer belt has important roles in the above-mentioned transferring step of the image forming process, such as transferring the toner image which is borne on the photoconductive element onto the transfer member, subsequently separating the transfer member from the photoconductive element, and continuously conveying the transfer member to the fixing device.
The transfer belt of the transfer-convey device is generally a seamless belt-shaped moving member made of an elastic material, such as a material containing rubber and having a medium resistance. The transfer belt is movably positioned around a plurality of rollers such as a drive roller, a driven roller and the like, and is positioned adjacent to the photoconductive element.
When transferring an image from the photoconductive element, the transfer belt that conveys the transfer member contacts the photoconductive element. The transfer belt conveys the transfer member which is sandwiched at a nipping position between the photoconductive element and the transfer belt. The transfer belt transfers the toner image on the photoconductive element onto the transfer member by use of a high voltage power source unit that applies a high voltage to a bias voltage applying roller disposed at a rear side of the transfer member.
After the transfer member passes through the above-mentioned nipping position between the photoconductive element and the transfer belt and, as a result, receives the toner image, the transfer belt that continuously conveys the transfer member causes the transfer member to be separated from the surface of the rotating photoconductive element. Then, the transfer belt conveys the transfer member to the fixing device.
During the above-described transferring step of the transfer-convey device, the transfer belt may have an undesirable deposition of toner particles on the surface thereof, particularly in the areas which are not covered by the transfer member. Such toner particles may eventually adhere to the transfer member and cause a dirty toner spot on the transfer member. Accordingly, to keep the transfer member held on the transfer belt clean, a cleaning blade is provided in the image forming apparatus. The cleaning blade is configured to remove the toner particles remaining on the surface of the transfer belt.
When a number of relatively small images are reproduced, or when a number of blank copies are erroneously reproduced in a copying machine, for example, which is provided with the above-described transfer-convey device, the amount of toner which is transferred from the photoconductive element to the transfer belt is reduced. As a result, the transfer belt may have almost no toner particles remaining thereon. Also, the cleaning blade that contacts the transfer belt may have almost no toner remaining on its edge portion. Accordingly, a friction coefficient between the transfer belt and the cleaning blade increases, that is, a frictional resistance between these two elements increases. As a result, the cleaning blade with its edge portion in contact with the transfer belt may be caught on a belt surface and may peel off and become damaged.
If the cleaning blade causes the peel-off problem in the edge portion thereof due to the above-described reasons, several serious problems may subsequently be caused. For example, the transfer belt cannot be cleaned sufficiently, which results in a rear face fouling problem in that the transfer member becomes dirty with deposited toner particles. A toner scattering problem may also occur around the mechanism of the transfer-convey device.
To solve the above problems, an additional toner supply can be provided to the above-described image forming apparatus. The surface of the photoconductive element is basically composed of an image forming area and a non-image area; the image forming area is in contact with the transfer member and the non-image area is in contact with the transfer belt, during the transferring step. In the non-image area, a line image is formed so as to supply toner to the transfer belt. Such a line image is referred to as a toner-line. The toner-line is transferred onto the transfer belt during the transferring step, and the toner-line supplies toner particles to the edge portion of the cleaning blade contacting the transfer belt. Thereby, the increase of the friction between the transfer belt and the cleaning blade can be prevented. Thus, the cleaning blade can be prevented from incurring the above-mentioned peel-off problem.
The toner-line may preferably be written in a full length direction parallel to the axis of the photoconductive element. Such a toner-line may preferably be formed in a frequency of, for example, once per several copies.
This toner-line is formed to include a greater amount of toner in comparison with the amount of toner that remains on the transfer belt during the transferring step. Therefore, the cleaning blade may have an excessive amount of toner at a time, depending upon how the toner is supplied to the cleaning blade. For example, if the toner-line on the transfer belt is a straight line parallel to the cleaning blade, the cleaning blade in its full length receives the excessive amount of toner. In this case, the cleaning blade cannot sufficiently scrape off the toner.
Further, the cleaning blade may be constructed so as to be capable of sufficiently cleaning the toner even in the above-described case. For example, the cleaning blade is adjusted to have a relatively high contact pressure to the transfer belt in order to strongly scrape off the toner. However, the thus-adjusted cleaning blade may excessively scrape off the toner during usual cleaning operations. Accordingly, the cleaning blade may deteriorate at an earlier stage than usual as a side effect of such an adjustment.
There is known a method for improving a cleaning ability of the cleaning blade to clean a belt-shaped photoconductive element, in which the cleaning blade is set to a relatively low contact pressure against the belt-shaped photoconductive element (Japanese Patent Laid-pen publication No. 60-107686). This method makes the cleaning blade contact the belt-shaped photoconductive element in a direction at an incline to a direction perpendicular to a driving direction of the belt-shaped photoconductive element. If this method is adopted to the cleaning blade of the transfer belt, the toner can be sufficiently removed from the transfer belt under the usual cleaning condition, even though the aforementioned toner-line is formed.
However, if the cleaning blade is constructed with an angle in the manner as described above, a relatively large space is required for mounting the cleaning blade. Also, the cleaning blade is required to be placed in a flat, but not round, surface of the transfer belt. Namely, this cleaning blade may not be applicable to areas where the transfer belt is wrapped around the drive roller, the driven roller and the like. This is because in the above-mentioned case a pressure of the cleaning blade is not uniformly applied to the transfer belt. This causes an insufficient cleaning of the transfer belt. Accordingly, it is not preferable to set the cleaning blade with the angle in the manner as described above since a setting position for the transfer belt is restricted.